Effects of minor alloying on the mechanical properties of Al based metallic glasses

Highlights

• Minor alloying with transition metals leads to an increase hardness of Al–Sm metallic glasses.

• Changes in topological short-range order cannot explain the increase in hardness.

• Hardening is due to strong bonding between Al and transition metals which increases resistance to local shear transformations.

• Effects of topology and chemistry on mechanical properties of metallic glasses can be independent of each other.

Abstract

Minor alloying is widely used to control mechanical properties of metallic glasses (MGs). The present understanding of how a small amount of alloying element changes strength is that the additions lead to more efficient packing of atoms and increased local topological order, which then increases the barrier for shear transformations and the resistance to plastic deformation. Here, we discover that minor alloying can improve the strength of MGs by increasing the chemical bond strength alone and show that this strengthening is distinct from changes in topological order. The results were obtained using Al–Sm based MGs minor alloyed with transition metals (TMs). The addition of TMs led to an increase in the hardness of the MGs which, however, could not be explained based on changes in the topological ordering in the structure. Instead we found that it was the strong bonding between TM and Al atoms which led to a higher resistance to shear transformation that resulted in higher strength and hardness, while the topology around the TM atoms had no influence on their mechanical response. This finding demonstrates that the effects of topology and chemistry on mechanical properties of MGs are independent of each other and that they should be understood as separate, sometimes competing mechanisms of strengthening. This understanding lays a foundation for design of MGs with improved mechanical properties.

Introduction

The unique combination of high strength and elasticity of metallic glasses (MGs) has attracted a lot of attention since the discovery of these materials [1,2]. Several MGs with a wide range of compositions have been synthesized to date [3] and minor alloying has emerged as common way to manipulate their mechanical properties [[4], [5], [6], [7], [8], [9], [10], [11], [12], [13]]. It is therefore important to understand how small amounts of alloying elements can have such a strong influence on the mechanical behavior of MGs.

Unlike crystalline metals, the mechanical response of MGs is not controlled by discernible structural features such as dislocations, stacking faults, and grain boundaries. The carriers of deformation in metallic glasses are called shear transformation zones (STZ) [14]. These are local clusters of atoms that undergo an inelastic shear distortion to accommodate plastic strains during deformation. STZs percolate along the planes of the maximum shear stress in the sample and eventually form regions of localized deformation called shear bands [2,15]. The activation of STZs and formation of shear bands is influenced by the atomic level structure in the MGs.

Even though MGs lack long-range structural order, they possess short- and medium-range orders, which can influence mechanical properties. Shi and Falk [16] studied the mechanical response of a binary metallic glass using the Lennard-Jones potential and proposed that metallic glasses derived their strength from a backbone of atoms with local short-range order (SRO). This report was followed by several other studies that investigated the influence of the atomic structure on the mechanical behavior of a more realistic system such as Cu–Zr and confirmed that indeed these MGs derived their mechanical strength from local SRO [6,[17], [18], [19], [20], [21], [22], [23], [24], [25], [26]]. Atoms arranged in icosahedral clusters were shown to be more resistant to shear transformations than other groups of atoms with geometrically unfavored motifs (GUMs). The GUMs are disordered regions with liquid like structure and act as soft spots with a lower resistance to shear transformations and fertile sites for activation of STZs [27,28]. A higher fraction of icosahedral clusters in the structure was therefore associated with a higher resistance to plastic deformation.

The effects of chemical bonding on the mechanical behavior of metallic glasses have been studied in the context of MGs with significant fractions (∼20% or more) of metalloids [[29], [30], [31]]. For example, in Pd–Si MGs, the covalent bonds associated with Si atoms were found to be more difficult to break than metallic bonds associated with Pd atoms. Since the fraction of Si atoms is significant in these alloys, Si atoms can suppress cavitation in Pd–Si, promoting a larger fracture toughness [29].

The effects of minor (i.e., on the order of a few percent) alloying elements on mechanical properties of MGs have been typically explained in terms of the effect that minor elements have on the atomic configurations and the topological order [[5], [6], [7], [8], [9],11,12]. For instance, the addition of small amounts of Al to Cu–Zr MGs has been shown to increase the resistance to plastic flow due to the strong, covalent-like bonding between Cu and Al atoms, which in turn leads to shortening of Al–Cu bond lengths and an increase in the icosahedral order [6]. In other words, the effect of the chemical bonding has been conflated with the increased topological order.

The goal of this work is to understand the role of minor alloying in the strengthening of Al rich MGs. Our interest in these MGs is motivated by their light weight, high specific strength [32,33], good corrosion resistance [34], and thermoplastic forming ability [35], which makes them excellent candidates for applications in such areas as micro- and nano-scale devices [35], wear and corrosion resistant coatings [36], composites with excellent mechanical and tribological properties [37]. We have chosen Al–Sm based glasses alloyed with different transition metals (TM = Cu, Ag, Au) for this study. The Al–Sm based glasses serve as a good model system since they are one of the better understood Al-based glasses [35,[38], [39], [40], [41]] and have been previously shown to possess good GFA and good stability due to the icosahedral SRO [40]. We have found that minor alloying of Al–Sm with TMs increases the strength of the MG, but this effect is due to the change in the chemical bond strength alone and cannot be explained by changes in topological order. In fact, we found that the effects of topology and the chemical bond strength are independent of each other in controlling the mechanical response of MGs subjected to minor alloying. Mechanical properties of the Al–Sm based glasses have been investigated experimentally using nanoindentation. The experiments are complemented by a detailed analysis of the atomic level structure using classical as well as ab initio molecular dynamics (MD).